As an input device for a user to input desired information, a keyboard with a plurality of keys is conventionally used. A keyboard provides great convenience for users who input desired information, but is of at least a certain size because the keyboard has a plurality of keys that are of a user-operable size. For example, a keyboard that has a QWERTY layout has at least 26 keys that correspond to each letter of the alphabet. However, in a mobile terminal such as a smart phone, since space for an input device is limited, a input device that is small in size is desirable. In such a mobile terminal, in order to reduce the size of an input device or increase an information display area on a touch panel into which an input device and a display device are integrated, the number of keys that may be operated at the same time is reduced. Reducing the number of keys means that a single key corresponds to a number of pieces of input information and therefore the number of key presses for a user to input given information increases. As a result, usability decreases.
Japanese Laid-open Patent Publication No. 2003-288156 discloses an input device that detects the position of a fingertip of an operator who operates a virtual keyboard with virtual keys arranged in a given keyboard layout and then generates a key code corresponding to one of the virtual keys where the fingertip was detected. With this input device, a digital video camera that records the operator's fingertips from the top and a digital video camera that records operator's fingertips from the front are used to detect the position of the operator's fingertips.